There is more free 2D art available than any one game could use. The constraint is not quantity, it is coherence, and that changes where you should look.
#Where to look
Kenney is the most useful single source in 2D game development. Thousands of sprites, tilesets, UI kits and icons, all CC0, and crucially all in one house style, so a game built entirely from Kenney packs looks deliberate rather than assembled.
itch.io has the best individual pixel art, much of it free or pay-what-you-want. Terms are per author: some state them clearly, some bury them in a readme, some say something friendly but vague. When it is vague, ask before you build a game on it.
OpenGameArt has a lot of everything under mixed licenses, and is where you go for the specific thing nobody else has.
Game-icons.net is the best source of UI and ability icons, CC BY, with thousands of them in one consistent line style.
#The problem nobody warns you about
Ten free packs from ten artists give you ten outline weights, ten palettes, ten pixel densities and ten ideas about what a shadow looks like. The result reads as free assets from across the room, before anybody notices a single sprite.
Three ways out, in order of how much work they are:
- Build from one source and accept its style as your art direction. This is why Kenney is worth more than its sprite count suggests.
- Recolor everything to one palette. Surprisingly effective, and a single afternoon. Pick eight to sixteen colors and force every sprite through them.
- Redraw the outliers. The slow answer, and sometimes the right one.
#Tilesets have a specific gotcha
A tileset only works if its tile size matches your grid and its edges are designed to meet the other tiles in the same set. Two tilesets that both say 32 by 32 will still not meet properly if they were drawn with different corner logic.
Check the transitions, not the tiles. The tiles always look fine.
#Keeping ten packs straight
The practical difficulty with 2D is volume. One pack can be eight hundred sprites, and eight hundred file names tell you nothing.
Tessera shows them as pictures, keeps each pack whole so you can tell which artist a sprite came from at a glance, and lets a collection gather the ones you actually chose across packs without copying a single file.
Install Tessera, or read collections.